The quality or state of being capable; competence, ability, or fitness to perform tasks.
From capable (from Latin capax meaning 'able to hold') plus -ness, a suffix forming abstract nouns. This compound structure became standard in English for creating quality/state nouns.
Capableness is one of those older English words that's been largely replaced by 'capability'—but 18th-century writers preferred 'capableness,' showing how language fashion changes even for basic concepts.
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